Present global mobile satellite communications systems, such as that disclosed in European Patent Application 0,365,885 A2 to Bertiger, et al. and IRIDIUM.RTM. Licensing Technical Information Document, Iridium LLC, both incorporated herein by reference, include interfaces with cellular telephone and data transmission equipped users, as well as interfaces with users of the public switched telephone network.
By way of example, a global mobile satellite communications system, as shown in FIG. 1, consists of, for example, satellites 50, 52, 54, 56 and related infrastructure including master control facility 60, back-up control facilility 62 and associated tracking, telemetry, and control facilities. The space portion utilizes a constellation of 66 operational satellites (of which only four are shown) in low-Earth orbit. The satellites 50, 52, 54, 56 are placed in, for example, six distinct planes in near polar orbit at an altitude of approximately 780 kilometers and circle the Earth.
The satellites 50, 52, 54, 56 are significantly closer to the Earth than geostationary satellites which orbit at nominal altitudes of 35,800 kilometers. The low orbit of the satellites 50, 52, 54, 56 enables the global mobile satellite communications system to achieve, with comparatively smaller main mission antennas, its link-margins permitting effective communications with portable, hand-held Individual Subscriber Units 60, 68 and other L-Band subscriber equipment 62, 64, 66 (hereinafter collectively ISUs).
Each satellite 50, 52,54, 56 communicates with ISUs 60, 62, 64, 66, 68 using main mission antennas and with other satellites in space using cross-link antennas. The global mobile satellite communications system operates, for example, in the frequency bands 1616-1626.5 MHZ for the subscriber links, 19.4-19.6 GHz and 29.1-29.3 GHz for the gateway earth station 70 links, and 23.1823.38 GHz for the inter-satellite links. The actual frequencies, within these ranges, used by the system are a matter of national regulatory approval and international frequency coordination.
Each satellite includes three phased array antennas, each containing an array of transmit/receive modules. The main mission antenna subsystem communicates with ISUs 60, 62, 64, 66, 68 through tightly focused antenna beams that form a continuous pattern on the Earth's surface.
FIG. 2 depicts a satellite beam footprint, wherein the beams 1-48 collectively produced by a single satellite combine to cover a substantially circular area with a diameter of approximately 4,700 kilometers. The global mobile satellite communications system architecture incorporates certain characteristics, such as call hand-off, which allow the space portion communications link with subscriber equipment to be transferred from beam to beam and from satellite to satellite, as such satellites move over the area where the subscriber is located.
The cross-link antennas permit satellites 50, 52, 54, 56 in the constellation to send traffic from one satellite to another. Each satellite has four cross-link antennas to allow it to communicate and route traffic to the two satellites that are fore and aft of it in the same orbital plane as well as neighboring satellites in the adjacent co-rotating orbital planes. Such inter-satellite networking provides benefits such as enhanced system reliability and capacity, and reduces the number of gateway earth stations 70 required to provide global coverage.
Inter-satellite networking provides access to the global mobile satellite communications system (to make or to receive calls) irrespective of gateway earth station location by routing a call from satellite to satellite until it is connected to the gateway earth station 70 that is most appropriate for the destination of the particular call. This feature also enhances the reliability of the service by permitting the global mobile satellite communications system to route calls around gateway earth stations 70 or satellites 50, 52, 54, 56 where required in the event of a malfunction, emergency, or other operational requirements.
The satellite constellation is monitored, managed and controlled by the system control facilities. The master control facility 60 is located, for example, in the eastern United States. The back-up control facility 62 is located, for example, in Italy. The tracking, telemetry, and control stations are located, for example, in northern Canada and Hawaii, and a transportable tracking station is located, for example, in Iceland. These facilities manage the performance and status of the individual satellites 50, 52, 54, 56.
The master control facility 60 also manages the network by developing and distributing routing tables for use by the satellites 50, 52, 54, 56 and gateway earth stations 70, directing traffic routing though the network, and controlling cell formation by the satellite main mission antennas. In addition, the master control facility 60 schedules gateway earth stations 70 to contact satellites and controls data flow between the master 60 and back-up 62 control facilities.
Gateway earth stations 70 provide call processing and control activities such as subscriber validation and access control for all calls placed in a gateway earth station territory. Each gateway earth station 70 also provides interconnection between Public Switched Telephone Networks (PSTNs) 80 and the global mobile satellite communications system by connecting calls made through the global mobile satellite communications system to and from the local PSTNs 80. Gateway earth stations 70 communicate with the space portion via gateway earth station link antennas on the satellites 50, 52, 54, 56 and ground-based antennas, or earth terminals, at each terrestrial gateway earth station facility.
Each gateway earth station facility 70 typically includes three antennas, a controller to manage communications with the constellation, an operations center to perform local network management, a paging message origination controller, and a switch that connects the gateway earth station to PSTNs 80 within the gateway earth station territory. As shown in FIG. 3, each gateway earth station 70 includes a visitor location register 130 used in call processing activities such as subscriber validation. Each gateway earth station 70 also keeps a record of all traffic in its territory and generates call detail records used in billing.
Call Processing in the global mobile satellite communications system consists of Acquisition, Access, Auto-Registration, Registration, Telephony, and Handoff.
Acquisition is the process of an ISU 60, 62, 64, 66, 68 obtaining a bi-directional communications channel, called a Traffic Channel, between the ISU and a satellite. The process is initiated either by the ISU user 60, 62, 64, 66, 68 taking some action to request a service that requires such a channel, or by the ISU 60, 62, 64, 66, 68 responding to a Ring Alert that notifies the ISU of an incoming call. Acquisition by an ISU 60, 62, 64, 66, 68 is necessary for registration, call setup, answering call terminations, or to initiate any service on the global mobile satellite communications system.
The Access process determines the location of the ISU 60, 62, 64, 66, 68 relative to Service Control Areas defined, for example, in earth fixed coordinates. Based on the Service Control Area within which the ISU 60, 62, 64, 66, 68 is found and on the identity of the ISU's access code, a decision is made regarding whether to allow service, and which gateway earth station 70 should provide that service. The Access process is initiated immediately following Acquisition.
Location information may be reported by the ISU 60, 62, 64, 66, 68 based on an external source such as Global Positioning System (GPS) or an aircraft navigation system, for example. Service is denied if the global mobile satellite communications system determines that the ISU 60, 62, 64, 66, 68 is in an unauthorized area.
Auto-registration refers to the capability of an ISU 60, 62, 64, 66, 68 to re-register with the network on an as needed basis. The ISU 60, 62, 64, 66, 68 automatically re-registers with the system when it knows its current location exceeds a specified distance from the point it last registered. In order to make this decision, the ISU 60, 62, 64, 66, 68 passively estimates both its location and its positional error, based upon information gathered from the ring channel of the passing satellites. The ring channel is, for example, a downlink-only channel, e.g., simplex, used to send Ring Alert messages to ISUs 60, 62, 64, 66, 68. Its downlink frequency is preferably globally allocated in order to be the same known frequency throughout the world. The ring channel uses, for example, a time division format to send Ring Alert messages to multiple subscriber units in a single frame.
Registration is the process of an ISU 60, 62, 64, 66, 68 communicating its location to the system, and requires the prior completion of the Acquisition and Access processes. The Registration process allows the network to maintain an estimate of the location of roaming users as part of mobility management. This location estimate is required to allow the network to notify the subscriber when an incoming call is available (e.g., `ring` an ISU 60, 62, 64, 66, 68 for a mobile terminated call). The ISU 60, 62, 64, 66, 68 must be registered in the gateway earth station 70 serving its location to initiate or terminate a call.
Telephony is the process of creating a connection between two telephones, at least one of which is an ISU 60, 62, 64, 66, 68, and of severing the connection at the end of the call. A call may originate or terminate at an ISU 60, 62, 64, 66, 68. Calls may be made between ISUs 60, 62, 64, 66, 68, or between an ISU and a Public Switched Telephone Network (PSTN) 80 subscriber. Multi-party services are also provided. Telephony includes the process of alerting an ISU 60, 62, 64, 66, 68 to an incoming call. Origination or termination of a call by an ISU 60, 62, 64, 66, 68 requires the prior completion of Acquisition and Access. Telephony protocols in the global mobile satellite communications system are, for example, patterned after the Global System for Mobile Communications (GSM) standard.
The global mobile satellite communications system satellites have highly directional antennas providing global mobile satellite communications system access to ISUs 60, 62, 64, 66, 68. These antennas are configured to project multiple beams onto the surface of the earth. Handoff is the process of an ISU 60, 62, 64, 66, 68 moving from its current Traffic Channel to a different Traffic Channel, usually because satellite motion has resulted in the current Traffic Channel no longer being suitable for continuing service. The handoff process is required in at least three situations:
1. An ISU 60, 62, 64, 66, 68 must be handed off between satellites as they move relative to the ISU (Inter-satellite hand-off). PA1 2. An ISU 60, 62, 64, 66, 68 must be handed off between beams on a satellite as beam patterns move relative to the ISU (Intra-satellite hand-off). PA1 3. As the inter-satellite geometry changes, radio channels are reallocated among the beams to manage interference. This process can cause an ISU 60, 62, 64, 66, 68 to be handed off to a different channel in the same beam (Intra-beam hand-off).
The satellite regularly provides updated lists of candidate beams for handoff, referred to as Candidate Beam Lists, to ISUs 60, 62, 64, 66, 68. A Candidate Beam List tells an ISU 60, 62, 64, 66, 68 which broadcast channels should be monitored in preparation for handoff. An ISU 60, 62, 64, 66, 68 typically initiates handoff when it detects that one of the candidate beams is likely to offer a better quality of service than the current beam.
The temporary ITU Document 8D-SRG/TEMP/6 (Rev. 1), incorporated herein by reference, was used to establish the appropriate required parameters for the global mobile satellite communications system, as well as the ETSI preliminary standard pr ETS 300 733, incorporated herein by reference.
Global mobile satellite communications system ISUs 60, 62, 64, 66, 68 comply with the applicable standards and requirements, including ITU and European Telecommunications Standards Institute (ETSI) standards as well as applicable FCC requirements. The FCC requirements include Part 25 of the FCC Rules, and Amendment of the Commission's Rules to Establish Rules and Policies Pertaining to a Mobile Satellite Service in the 1610-1626.5/2488.5-2500 MHZ Frequency Bands, Report and Order, 9 FCC Rcd 5936 (1994), all of which are hereby incorporated by reference.
Many of the system parameters associated with the ISU-satellite L-Band interface have been described previously, but are repeated here for convenience. Additional parameters and characteristics associated with ISUs 60, 62, 64, 66, 68 are provided below.
The L-Band interface is designed with, for example, an FDMA/TDMA/TDD system architecture and with an FDMA channel separation of 41.666 kHz in the 1616 to 1626.5 MHZ frequency band. The TDMA/TDD structure is based on a 90 milli-second frame and is composed of a 20.32 milli-second simplex time-slot, followed by four 8.28 milli-second uplink time slots and four 8.28 milli-second down link time-slots, with various guard times interspersed. The modulation used is, for example, DEQPSK, with square root raised cosine filtering using a rolloff factor of 0.4. The data rate is, for example, 50 kbps. The occupied bandwidth (unless otherwise permitted by the ITU definition) preferably does not exceed 31.5 kHz. The FCC authorized bandwidth is 41.67 kHz. The ITU emission designator is 41k7Q7W.
ISUs 60, 62, 64, 66, 68 are capable of operating from 1616.0-1626.0 MHZ; however the actual frequencies used are in accordance with regional spectral licenses and international frequency coordination.
The ISU transmitter frequency stability preferably does not exceed approximately 1.5375 ppm, 1.5 ppm being typical. The ISU permitted frequency deviation preferably does not exceed approximately 26.3 ppm, based on 37.5 kHz maximum Doppler and 5 kHz frequency accuracy.
The ISU antenna uses, for example, Right Hand Circular Polarization, and provides a maximum gain of approximately 3.5 dBic from 8.2 to 90 degrees elevation, and a maximum gain of approximately 0 dBic at 0 degree elevation.
In accordance with the Final Acts of the 1995 World Radio Conference (WRC-95), the maximum Effective Isotropic Radiated Power (EIRP) transmitted by an ISU 60, 62, 64, 66, 68, averaged over a 90 milli-second frame, does not exceed approximately -3 dBW/4kHz within any sub-band of the band in which it is intended to operate. ISU transmissions are power controlled over a minimum range of approximately 8 dB in 1 dB increments.
The ISU G/T is a maximum of approximately -20.5 dB/K, based on a 250 K receiving system noise temperature with a maximum gain of approximately 3.5 dBiC.
I have determined that it would be desirable to have a system and/or a method for effectively increasing the number of subscribers that can be subject to a call intercept in a global mobile satellite communications system without substantially increasing the effective size of a telephony intercept list.
I have determined that it would be desirable to have a system and/or a method for minimizing the amount of call set-up time needed to verify whether any call is subject to a call intercept in a global mobile satellite communications system without effectively increasing the size of the telephony intercept list.
It is also desirable to have a system and/or method that is capable of handling an increased number of subscribers subject to a call intercept in a global mobile satellite communications system.